FIG. 1 is a schematic block diagram of a wire bonding apparatus disclosed in Japanese Published patent application no. 57-39055. In this apparatus, a wire 1 coiled on a wire spool 5 is fed through a clamp 4 and a capillary 2. A ball 1a is formed at the end of the wire 1 projecting out of the capillary 2 in an arc discharge. The arc is formed between the end of the wire protruding from the capillary 2 and an electrode 3. A high voltage is applied between the electrode 3 and the wire 1 which are intended to be separated by an air space. A resistance Rg represents the resistance of the air gap. A discharge voltage circuit 6 applies a voltage of at least 2,200 volts between the electrode 3 and the end of the wire, producing a discharge that melts the end of the wire. The molten metal of the wire forms a sphere or a ball la because of the surface tension of the molten metal. A discharge current supply circuit 7 supplies a constant discharge current between the electrode 3 and the wire 1, once an arc is established, for a time period determined by a discharge time setting circuit 8.
In order to form a ball at the end of the wire 1, a start signal is supplied externally and, in response, a drive circuit 9 delivers a start signal both to the discharge voltage circuit and the discharge time setting circuit 8. A gap voltage detection circuit 10 detects the voltage across the gap between the electrode 3 and the wire 1 and a voltage determining circuit 11 determines whether the voltage across the gap exceeds or is less than a preestablished reference voltage. An output means 12 produces a signal in response to the determination by the voltage determining circuit 11 of the relative magnitude of the gap voltage. The output signal corresponding to this comparison is supplied to a display (not shown) for displaying information on whether the discharge is being conducted properly.
The time sequence of operation of the apparatus of FIG. 1 is illustrated in FIGS. 2(a)-2(h). At time t.sub.1, the drive circuit 9 delivers a start signal to the discharge voltage circuit 6 and to the discharge time setting circuit 8, as shown in FIG. 2(a). The discharge voltage circuit 6 supplies a voltage of at least 2,200 volts for a period of about one millisecond, as illustrated in FIG. 2(b), to initiate a discharge between the wire 1 and the electrode 3. In response to the start signal, the discharge time setting circuit 8 supplies an operational signal to the discharge current supply circuit 7 from time t.sub.1 to t.sub.2, as shown in FIG. 2(c). During that time period, the discharge current supply circuit 7 supplies a constant discharge current I.sub.s, as shown in FIG. 2(d). The continued flow of the discharge current during this time period results in the melting of the end of the wire 1 and the formation of the ball 1a. At the same time, the voltage across the gap between the wire 1 and the electrode 3 is monitored by the gap voltage detection circuit 10 which, during proper operation, has the waveform shown in FIG. 2(e). After a predetermined time delay between the times t.sub.2 and t.sub.3 when the discharge initiating voltage is applied across the gap, a sustaining voltage is applied across the gap. The sustaining voltage is supplied to the voltage determining circuit 11 where a comparison is made between the voltage across the gap and a reference voltage V.sub.0, as shown in FIG. 2(f). The voltage maintained across the gap may be larger or smaller than the reference voltage, as illustrated in FIGS. 2(g) and 2(h), respectively. The result of this comparison is supplied to a display section (not shown) to indicate whether the discharge is being properly carried out.
In the course of forming a ball 1a, as illustrated in FIG. 3, the heat energy HB in calories required to form the ball can be calculated from the formula: EQU HB=Rg(Is).sup.2 t.sub.s /4.185=Vg Is t.sub.s /4.185
where Rg is the gap resistance, Vg is the gap voltage, Is is the discharge current, and t.sub.s represents the discharge time. When the heat energy required to form a particular ball is known, interrelated values of Vg, Is, and t.sub.s can be determined.
In the idealized operation of the prior art apparatus described above, the gap voltage Vg, the discharge current Is, and the discharge time t.sub.s are all constant. However, in operation, as illustrated in FIG. 4, the apparatus does not always operate as intended. In the desired operation, the wire 1 is extended by a length l.sub.1 from the capillary 2, leaving a gap of length l.sub.2 between the tip of the wire and the electrode 3. However, when the length l.sub.1 of the wire varies, the discharge distance, i.e., the gap of length l.sub.2, varies, altering the gap resistance and the voltage required to initiate a discharge. Likewise, the total heat energy supplied to the wire in the discharge with controlled parameters can change, making it extremely difficult to repeatedly form balls at the end of the wire that are uniform in size. Size uniformity of the balls is extremely important in the manufacture of semiconductor devices where the balls are used to make ball bonds to electrodes of semiconductor devices. As the density of leads extending from semiconductor devices has increased in recent years, the space available for each ball bond has been reduced. The reduced spacing has increased the criticality of reliably and repeatedly forming balls of essentially uniform size.
Ball size non-uniformities can be caused by mechanical variations in the operation of the bonding apparatus. For example, when the wire 1 is not advanced through the clamp 4, no discharge can be sustained, even if the discharge can be initiated between the capillary 2 and the electrode 3. On the other hand, when the wire 1 is advanced too far through the capillary tube, it may actually touch the electrode 3 so that a discharge cannot be formed between the wire 1 and the electrode 3 and no ball can be formed on the end of the wire. Even when the wire is advanced as desired, as illustrated in FIG. 4, vibrations may be present that can significantly alter the relatively small gap that is present between the wire 1 and the electrode 3. These variations in the gap length l.sub.2 interfere with the formation of balls of uniform size.